Internal combustion engines generally are of two types: a reciprocating piston or a rotary piston. In the reciprocating piston engine, the fuel may be gasoline or diesel. Conventional gasoline engines may be either four stroke or two stroke, both of which burn the fuel so as to release thermal energy, which is then converted into mechanical energy. In the reciprocating piston gasoline engine, a mixture of gasoline and air is drawn into a cylinder, compressed by the piston, and ignited by a spark. In a diesel engine, air alone is drawn into the cylinder and is compressed to a higher ratio than in a gasoline engine, thereby increasing the air temperature, after which diesel fuel is injected into the cylinder for spontaneous ignition.
A rotary piston engine operates on the same fuel combustion principles as the gasoline engine, but utilizes a rotary piston, instead of an oscillating piston which has to be alternately accelerated and decelerated in the cylinder. Thus, the rotary piston engine obviates the forces of inertia associated with the reciprocating piston engine, resulting in higher rotational speeds. As with the reciprocating piston engine, the rotary piston engine includes the steps of drawing in the fuel and air mixture, compressing the mixture, combustion, and discharge of the burned gasses. One of the major problems with rotary engines is the seal of the three combustion chambers in relation to one another.
In a conventional four stroke reciprocating piston engine, energy is produced during the third power stroke, with energy being required for the intake and compression strokes, and for the gas expulsion stroke.
A turbine is another form of rotary engine actuated by the reaction or impulse of a current of fluid, such as water, steam or air, subject to pressure, and usually made with a series of curved veins on a central rotating shaft. In a steam turbine, the energy of steam under pressure is used for producing a mechanical rotary motion, which is often used for electric power generation. Gas turbines are driven by the combustion gasses to produce a rotary motion by deflecting the combustion gasses with rings of blades mounted on a rotor. The gas or air turbine cannot transmit its entire power output to the generator, since a substantial portion of the power is required for driving the compressor.
Internal combustion engines, as compared to turbines, are more efficient in converting thermal energy into mechanical energy. The prime reason of higher efficiency is the high compression ratio, which leads to a high combustion temperature. The fuel mixture in the combustion chamber cylinder wall and piston are exposed to very high temperature gradient for a brief time period followed by quick cooling by the introduction of fresh charge, thereby avoiding degradation of the cylinder and piston material. On the other hand, turbines do not have any reciprocating parts, and therefore run at very high speeds. Consequently, turbines impart high power output per unit weight. Unlike internal combustion engines, only the inlets of turbines are subjected to continuous highest temperatures equal to the combustion temperature. Therefore, the thermal efficiencies of turbines are limited by the maximum withstanding temperatures of the inlets. Steam turbines also run at high speeds, operate on inexpensive residual fuels and deliver high power output. However, they require enormous amount of water for cooling the condenser, which make them suitable for power stations.